Packet switching systems of both the distributed and non-distributed type are known. Distributed type systems typically use a single conductor or co-axial cable that is connected to all system ports in a bus or ring configuration. A transmitting port, in these systems, communicates with a receiving port by contending for bus accesses, by winning contention, and by then transmitting its data to the receiving port. The transmitted data includes the address of the receiving port plus other header or setup information.
Although bus and ring type systems are conceptually simple, they have a number of disadvantages. For example, their reliability is not high since the entire system becomes inoperative if the single conductor bus is cut or broken. Also, a single malfunctioning port can monopolize the use of the bus by applying a continuous string of meaningless data to the bus. This can render the entire system inoperative. Further, the system throughput is not high since the single conductor bus must be used for all system functions such as arbitration, the exchange of control signals, as well as for the transmission of data between ports. Data security is also poor since the cable is long and directly extends to all user stations. This renders it susceptible to bugging by unauthorized personnel.
Non-distributed type packet switching systems, such as that shown in, U.S. Pat. No. 4,470,112 of Sept. 4, 1984 to J. O. Dimmick overcome the above mentioned disadvantages of the bus or ring type single conductor systems. The Dimmick data bus length is considerably shorter than that of the bus or ring type systems. This permits a higher bus frequency response. The ports are adjacent the data bus. The system is therefore relatively compact and can fit into a single location such as a secure equipment closet or the like. Lines extend from the ports to the associated user stations. This provides better security against bugging since the data bus itself is not accessible.
The Dimmick type system also has separate arbitration and data buses. This increases the sytem throughput by permitting bus arbitration and the data transmission to take place concurrently.
Although the Dimmick non-distributed type system is an improvement over the bus type, it still has disadvantages. For example, it requires a costly and complex central switch to convey data from a sending to a receiving port. On each packet served by the system, the transmitting port sends to the switch the data to be transmitted together with information indentifying the transmitting port and station. The switch translates the received calling station address into the address of the destination port and station. This translated information is inserted by the switch into the received packet so that the packet can be transmitted to the destination port and station.
The use of a centralized switch is costly. It is further disadvantageous in that in inhibits system flexibility and expansion. The size of the switch defines the maximum number of ports that can be served by the system. This number cannot be exceeded without replacing the existing switch with one of a higher port capacity.
Systems of the type shown by Dimmick also typically use the data bus to transmit data as well as to transmit control signals between the central controller and the ports and switch. This decreases system reliability and through-put under conditions in which a manfunctioning port is monopolizing the use of the data bus. This prevents the controller from sending commands over the data bus to deactivate the malfunctioning port.